Many cooling systems comprising metallic parts which come into contact with circulating fluids are subject to corrosion of those metallic parts. Water which has excellent heat transfer characteristics is a common cooling fluid used for such systems. However, where there is a concern that water might be subject to freezing conditions, polyhydric alcohols such as ethylene glycol, propylene glycol, and glycerol, or mixtures of water and polyhydric alcohols, are typically employed. While the alcohols are not inherently corrosive to metals, they are normally diluted with water to form the cooling fluid, or are at least exposed to moisture in use. Aeration of the aqueous fluid during use tends to induce corrosive conditions in the fluid which can become quite severe after prolonged use. In addition, rapid fluid flow or vibration can produce cavitation which occurs when flow conditions result in rapid formation and collapse of vapor pockets in the flowing liquid in regions of very low pressure. The resulting high localized shock forces erode protective metal oxide films and accelerate corrosion. Cavitation damage primarily occurs in components made of cast iron, aluminum, and their alloys.
Alcohols such as ethylene glycol, propylene glycol, and diethylene glycol are used as a nonvolatile, permanent-type antifreeze and high temperature transfer fluid in liquid-cooled automotive and stationary internal combustion engines to prevent freezing and overheating and damage to the engine water jacket. The most important property of an engine antifreeze formulation after heat transfer and freezing point depression characteristics is its ability to prevent corrosion in the cooling system. An automotive cooling system contains a variety of metals that are subject to corrosion and/or cavitation such as copper, solder, brass, steel, cast iron, and aluminum. Rust or other solid matter suspended in the coolant may cause erosion damage at points of high coolant velocity. The presence of oxygen and the high temperatures, pressures, and flow rates in automotive cooling systems increase the possibility of erosion and corrosion attack. Cavitation damage may also be a particular problem, for example, in the water pump, cylinder liners, crankcase, and radiator.
Various combinations of inorganic and organic inhibitors have been added to cooling fluids to inhibit corrosion and cavitation and reduce damage to metallic surfaces. There are several difficulties in selecting an effective inhibitor combination for a given system. Each type of metal generally requires a separate corrosion inhibitor. For example, a given inhibitor may be effective to reduce corrosion of ferrous metals, but does not provide effective protection against corrosion of non-ferrous metal components of the system. Further, many conventional corrosion inhibitors are often ineffective in protecting cast iron and aluminum against cavitation, or protect cast iron against cavitation but do not protect aluminum and aluminum alloys against corrosion. Some cast iron cavitation inhibitors may even cause increased corrosion of aluminum and aluminum alloys. Certain cavitation inhibitors may only be effective at high concentrations. Phosphate-based corrosion inhibitor formulations (i.e. formulations where phosphate is an important component of the inhibitor) and borate-based corrosion inhibitor formulations (i.e. formulations where borate is an important component of the inhibitor) are two common types of inhibitor systems employed for cooling system treatment. Typically the formulations contain, in addition to the base components (i.e. phosphate and/or borate), other agents such as corrosion-inhibiting and cavitation-inhibiting compounds, and where desired, deposit control agents, impingement (i.e. erosion) inhibitors, alkalinity control agents, and surfactant compounds including detergents and antifoaming agents, so that the effectiveness for a particular application may be optimized. While phosphate and borate are themselves considered to be pH stabilizing, other pH stabilizing agents may also be employed.
The cooling systems of many liquid cooled engines, such as those used in certain heavy-duty trucks, function for long periods of time without coolant change; and filters may be employed within the engine cooling systems for filtering solid impurities from the circulating coolant. Corrosion inhibitors are generally used to retard the corrosive effect of the circulating liquid on the metallic parts of the cooling system which it contacts. Over long periods of time, however, corrosion inhibitors can react, degrade, or otherwise lose their effectiveness and supplemental dosages must be added. Supplemental cooling additives are discussed, for example, in U.S. Pat. No. 4,717,495.
One method of adding corrosion inhibitors to cooling systems with filters is to provide filters with corrosion inhibitor dosages within so that the corrosion inhibitor is added with the filter change. Typically, in this process, powders or briquettes of corrosion inhibitor solid are loaded manually into the filter. Phosphate or borate-based inhibitors have been used for this purpose with some success. More recently, however, there has been an effort to develop slurries which set under normal manufacturing and storage conditions but are pumpable such that they may be pumped into engine filters and thereafter loaded with the filters into the cooling system. Pumpable slurries are designed to eliminate the need to use solid powders, briquettes, or the like, and to set upon storage both within the filter before the filter is installed, and within any storage vessel from which it is pumped into the filter, so that the potential for running of liquid is minimized. Naturally, a suitable slurry of this type should provide the desired corrosion protection to the engine cooling system after it is loaded into the system with the cooling system filter. It is also desirable that the slurry components are compatible with the cooling fluid.